Many forms of enclosed environment require the removal of practically all of the suspended particulate material from the air. The manufacturer of delicate instruments commonly requires such conditions, and hospitals and laboratories have similar needs. Standard replaceable filter elements have been developed to provide the necessary high degree of filtration, and these are used in conjunction with air duct systems that terminate in housings providing receptacles for the installation of these filter elements. These housings are often incorporated in the ceiling structure of work rooms, with the result that the outlet openings from the housing form a very large percentage of the area at the top of the room. The installation and replacement of the filter elements have been made possible from a position within a room by structures of the types shown in U.S. Pat. Nos. 4,088,463 and 4,217,121. It is very important that the replacement of the filter elements should disturb as little structure as possible to avoid the discharge of accumulated dust particles into the room. This obviously applies also to the installation and maintenance of electrical circuitry, including the lighting system. Existing "clean air" installations involve a complete separation of the lighting system from the filtration units, resulting in the decrease in available ceiling space for the filtration units, and a disturbance of considerable normally fixed structure in the course of possible maintenance of the entire electrical installation. Fire-control sprinkler systems are also commonly treated as entirely separate installations, having the additional effect of complicating the space requirements where the maximum filtration is desired.